The present invention relates generally to a method and an electrode for welding a stranded metal workpiece to a metal substrate workpiece using electrical-resistance welding and to a weldment article of manufacture made thereby. More particularly, the invention relates to a method and electrode for welding a stranded copper lead wire to an aluminum substrate of an electrical connection such as, for example, a transformer tap, and to the weldment article of manufacture made thereby. It is to be appreciated, however, that the invention has broader application and, in that regard, is applicable to welds and welding of any selected metal stranded workpieces to metal substrate workpieces for use in electrical, mechanical or any other connections.
For aiding in understanding of the present invention and an application thereof, reference is made first to FIGS. 1 and 3 which show an application particularly well suited for use of the present invention and to an electrical connection formed in accordance with the prior art, respectively. FIG. 1 is an illustrative schematic view showing a typical industrial power supply 10 of the type including a multi-tap transformer 12 connected with a rectifier 14 through an intermediary electrical connection network 16. In systems of the type illustrated, it is desirable to provide the rectifier 14 with various voltage levels which are readily derived from multiple taps 18a-18f of the transformer 12. A switch device 20 includes a wiper arm 22 of the other switch means which may be moved into selected positions for connecting the rectifier 14 to individual transformer taps 18a-18f at various voltage levels through intermediary lead wires 24a-24f. 
Of particular relevance to the present application, however, the lead wires 24a-24f are typically stranded copper lead wires and the plurality of transformer taps 18a-18f are typically provided in the form of individual aluminum substrate members. More particularly, with reference to FIG. 2, a single lead wire 24 includes a jacketed portion 26 and a stripped portion 28 attached with an aluminum substrate transformer tap 18 in accordance with the present application. FIG. 2A shows a cross-sectional schematic view of a typical lead wire 24 which includes an outer insulative sheath 30 surrounding an inner stranded core 32 formed of a plurality of individual conductive copper strands 34.
In accordance with the prior art, connections between copper flex leads 24 and aluminum transformer tabs 18 were made using a soldering technique. As best shown in FIG. 3, the individual copper strands 34 of the stranded core 32 were soldered to the aluminum substrate 18 typically utilizing a cadmium/zinc alloy solder 38. One problem with the prior art soldering process, however, is that it requires the flex leads to be tinned before the solder is applied. To that end, corrosive fluxes must be used in order to prepare the mating surfaces. Oftentimes, water is needed to cool the connection after the soldering process. The tinning and cooling steps are time consuming. Still further, the prior art soldering process illustrated in FIG. 3 is highly dependent on human technicians which adversely affects the consistency of the connections.
Resistance spot welding is widely used in many automated processes as an alternative to soldering for joining steel and other materials, particularly in the assembly of automobile bodies, consumer products, and the like. An apparatus for resistance spot welding includes at least one resistance welding electrode which is pressed against a pair of superposed workpieces whereupon an electric current is applied thereto. With the electrodes placed against opposing sides of the workpieces to be welded, the electric current is passed through the electrode and the workpieces. The electrical resistance of the metal workpieces produces localized heating which causes the workpieces to fuse at the weld site. The electrical heating at the point of pressure of the electrode forms a molten nugget at the interface between the workpieces. The nugget freezes after the electrical current is removed.
Electrodes are typically made from copper or copper alloys to provide low electrical resistivity and allow high current flow therethrough. The tip of the electrode in contact with the workpiece typically has a geometrical configuration which is determined by the particular needs of the welding process. In most applications, the steel workpieces are somewhat forgiving in nature and the shape of the electrode, pressure applied between workpieces, and current applied thereto are parameters easily selectable to provide suitable results.
However, heretofore, electrical resistance welding has not been used to join a stranded copper workpiece with an aluminum substrate workpiece, primarily because of difficulties in properties exhibited by aluminum during phase transition. Parameters such as contact pressure, current intensity and duration, and electrode tip configuration needed to produce desired results have eluded those skilled in the welding arts.
In accordance with the above, therefore, there is a need in the art for a welding method and an electrode apparatus for welding a stranded metal workpiece to a substrate metal workpiece using electrical-resistance welding techniques, and to a weldment article of manufacture made thereby. More particularly, there is a need for a method and electrode for welding a stranded copper lead wire to an aluminum substrate and to the weldment article of manufacture made thereby.